Devices powered by a single alternating-current (AC) or direct-current (DC) power supply do not have much portability. While battery-driven devices are portable, the limited energy capacity of batteries leads to inconvenience of use. A device that can be powered by both a power supply and a battery is configured in such a way that the power supply and the battery are connected in parallel to a load system and that at least a diode is connected in series between the battery and the load system. If the battery is rechargeable, a charger is additionally provided so as to charge the battery while an AC or DC power supply is being used. Thus, the battery has its service life extended, and the frequency of battery change is reduced. Recently, it is common practice to add a power management device to a battery charger to control the charging current to the battery, thereby not only increasing charging efficiency and prolonging the battery's service life, but also providing enhanced protection to the battery. However, when applied to a device that is powered by both a power supply and a battery, this type of battery charger may result in reverse current. Since the service life of a rechargeable battery is related to the number of times the battery has been charged and discharged, the battery may die earlier than it should due to the reverse current. Referring to FIG. 1, a device that is powered by both a power supply and a battery includes two current paths for connecting an AC power supply ACIN and a battery 10 to a load system in parallel. The load system typically has a power converter for converting the voltage V4 at a power output OUT of the power system into an appropriate voltage. A MOS transistor M3 controls the input current supplied by the AC power supply ACIN, an over-voltage protection (OVP) device 12 will turn off the MOS transistor M3 when the voltage V2 at a power input IN of the power system is too high, and a parasitic diode D3 of the MOS transistor M3 has a cathode connected to the power input IN so that the MOS transistor M3, when turned off, completely cuts off the current from the AC power supply ACIN. A MOS transistor M1 controls the charging current that flows to the battery 10, and a power management device 14 detects the voltage V6 at the charging output BATT to turn off the MOS transistor M1 when the voltage V6 exceeds a preset value, thus preventing the battery 10 from being overcharged. The MOS transistor M1 has a parasitic diode D1 whose cathode is connected to the power output OUT to allow current flowing from the battery 10 to the power output OUT when the MOS transistor M1 is turned off. In this circuit, however, whenever the voltage V2 at the power input IN becomes lower than the voltage V6 at the charging output BATT, a reverse current flows from the battery 10 to the AC power supply ACIN. Even if the MOS transistors M1 and M3 both are turned off, the reverse current can flow to the AC power supply ACIN through the parasitic diodes D1 and D3, thus reducing the service life of the battery 10.
To prevent this reverse current, a diode D4 is connected to the power input IN in series. However, the diode D4 will cause the voltage V4 to be lower than the voltage at the AC power supply ACIN by a forward bias of the diode D4 and thereby reduce the efficiency of charging the battery 10.
As an alternative approach to preventing the above-illustrated reverse current, a MOS transistor M2 is connected in series between the MOS transistor M1 and the charging output BATT, and the MOS transistor M2 has a parasitic diode D2 whose cathode is connected to the charging output BATT. This approach requires relatively complex control. When the MOS transistor M2 is turned off, the battery 10 provides no current, but as long as the voltage V2 at the power input IN is higher than the voltage V6 at the charging output BATT, the MOS transistor M1 still can be controlled to charge the battery 10. When the MOS transistor M2 is turned on, and the voltage V2 at the power input IN is lower than the voltage V6 at the charging output BATT, current is supplied by the battery 10, and yet a reverse current also flows to the AC power supply ACIN. In order to prevent any such reverse current from flowing to the AC power supply ACIN, it is again necessary to connect the diode D4 in series. Also, the addition of the MOS transistor M2 lowers the efficiency. When the battery 10 is being charged, the voltage V6 is lower than the voltage V4 by a voltage drop of the MOS transistor M2 or its parasitic diode D2. On the other hand, when the battery 10 supplies current, the voltage V4 is lower than the voltage V6 by a voltage drop of the MOS transistor M2. Those voltage drops result in lower efficiency.
In addition, the use of the diode D4 or the MOS transistor M2 increases the size and cost of the circuit. More particularly, both the diode D4 and the MOS transistor M2 are located in a current-supplying path to the load system and thus, they must be power components in support of the resultant large current. The power components, which are large and expensive, significantly increase the size and cost of the circuit. Nowadays, all these power components are external separated components that cannot be integrated into a controller chip, making it impossible to reduce the size and cost of the whole circuit.
The problems mentioned above also exist in other similar systems such as those using an AC/DC converter or a USB-based DC power supply in place of the aforesaid AC power supply ACIN.